A projection type display device mainly comprises a light source, color light separating means for separating a white color luminous flux emitted therefrom into color luminous fluxes of three primaries, three liquid crystal light valves for optically modulating separated luminous fluxes of each color, color light combining means for combining modulated luminous fluxes of the optically modulated colors through these liquid crystal light valves, and a projection lens for magnifying and projecting the combined modulated luminous flux on a screen.
FIG. 8 is a conceptual diagram showing a basic constitution of a conventional projection type display device. Referring to FIG. 8, the process until an image is magnified and projected on a screen is described.
A natural light emitted from a light source 201 passes through an illuminating optical system composed of a first integrator 202a, a reflection mirror 207, and a second integrator 202b, and goes toward a first dichroic mirror 203 which passes blue and reflects green and red.
The blue light passing through the first dichroic mirror 203 is reflected by a reflection mirror 207B, focused by a field lens 205B, and reaches modulating means 100B. On the other hand, of the green light and red light, the green light is reflected by a second dichroic mirror 204 which reflects green and passes red, and is focused by a field lens 205G, and reaches modulating means 100G.
The red light passes through the second dichroic mirror 204, and is focused by a field lens 205R through first relay lens 206a, reflection mirror 207R, second relay lens 206b, and reflection mirror 207R, and reaches modulating means 100R. Herein, the color lights modulated by the modulating means 100B, 100G, 100R according to the video signal corresponding to the colors are put into a dichroic prism 105.
FIG. 7 is a detailed diagram of modulating means and color light combining means shown in FIG. 8. As the color light combining means, generally, the dichroic prism 105 is used, and this dichroic prism 104 is composed by mutually gluing four triangular prisms of same shape. Moreover, on the X-shaped glued surface, a reflection layer of dielectric film or the like having a selective reflection characteristic (or selective transmission characteristic) of a specific color is formed, and a blue light reflection plane 1051B and a red light reflection plane 1051R are provided.
Color lights modulated according to the video signal corresponding to the colors by the modulating means 100B, 100G, and 100R composed of incident side polarizer 107, liquid crystal light valve 101, and exit side polarizer 102 are combined by the dichroic prism 105 (color light combining means), and projected on a screen (not shown) through a projection lens 106 which is projection optical means.
Herein, the modulated luminous flux of each color passing through the modulating means 100B, 100G, 100R either passes through the dichroic prism 105, or is reflected by the blue light reflection plane 1051B or is reflected by the red light reflection plane 1051R for composing the X-shaped reflection plane, and is emitted to the side of the projection lens 106.
However, a slight portion of the light is not reflected by blue light reflection plane 1051B or the red light reflection plane 1051R for composing the X-shaped reflection plane, but passes through, and may reach up to the back side of the liquid crystal light valve 101 for composing the modulating means 100B, 100R confronting each other across the dichroic prism 105.
For example, the blue modulated luminous flux passes through the blue light reflection plane 1051B of the dichroic prism 105, leaves the red luminous flux incident plane 105R, and enters the back side of the liquid crystal light valve 101 of the red color modulating means 100R.
Similarly, the red modulated luminous flux passes through the red light reflection plane 1051R of the dichroic prism 105, leaves the blue luminous flux incident plane 105R, and enters the back side of the liquid crystal light valve 101 of the blue color modulating means 100B.
Furthermore, the green modulated luminous flux may be slightly reflected by the red light reflection plane 1051R or blue light reflection plane 1051B of the dichroic prism 105, and may enter the back side of the liquid crystal light valve 101 of the red color modulating means 100R and the back side of the liquid crystal valve 101 of the blue color modulating means 100B.
In this way, if the light enters from the back side of the liquid crystal light valve 101 into the liquid crystal light valve 101, the liquid crystal light valve 101 may malfunction and other adverse effects may be caused.
Thus, it is a first problem that the light emitted from modulating means for other color enters the back side of the liquid crystal light valve 101 for each color as leak light from the dichroic prism 105 to cause adverse effects on the liquid crystal light valve 101, and in particular the effect is greater by the blue light which is light at the shorter wavelength side.
Moreover, the modulated luminous fluxes of red, blue and green passing through the modulating means for each color may be slightly reflected by the incident planes 105B, 105G, 105R of each dichroic prism and the exit side polarizer surface 102S, and may enter each liquid crystal light valve 101 from the back side as return light.
Such return light is one of the factors to cause malfunction of the liquid crystal light valve 101 as mentioned in the first problem, and hence it is a second problem.
To solve these problems, hitherto, it has been proposed to install an absorption type filter between the color combining means and modulating means as disclosed in Japanese Laid-open Patent No. 8-184797. The absorption type filter is, however, small in the degree of freedom for setting the spectral characteristic, and is not sufficient to prevent the harmful light completely depending on the color light. If composed to prevent completely, the transmission rate of the usable color lights is decreased.
It is hence an object of the invention to present a projection type video image display device comprising an optical system capable of solving such problems.